1. Field of the Invention
The invention relates to a device for spatially dividing and combining power of an EM wave using a plurality of longitudinally parallel trays. More particularly, the invention relates to a device for dividing and combining the EM wave by antipodal finline arrays provided within a coaxial waveguide cavity.
2. Description of the Related Art
The traveling wave tube amplifier (TWTA) has become a key element in broadband microwave power amplification for radar and satellite communication. One advantage of the TWTA is the very high output power it provides. However, several drawbacks are associated with TWTAs, including short life-time, poor linearity, high cost, large size and weight, and the requirement of a high voltage drive, imposing high voltage risks.
Solid state amplifiers are superior to TWTAs in several aspects, such as cost, size, life-time and linearity. However, currently, the best available broadband solid state amplifiers can only offer output power in a watt range covering about 2 to 20 GHz frequency band. A high power solid state amplifier can be realized using power combining techniques. A typical corporate combining technique can lead to very high combining loss when integrating a large amount of amplifiers. Spatial power combining techniques are implemented with the goal of combining a large quantity of solid-state amplifiers efficiently and improving the output power level so as be competitive with TWTAs.
U.S. Pat. No. 5,736,908, issued to Alexanian et al., discloses a power combining device using a slotline array within rectangular waveguides. In an embodiment shown in FIG. 7 of that patent, a circular waveguide is shown, but the slotline array is arranged with elements that are disposed in parallel within the waveguide.
In N. S. Cheng, Pengcheng Jia, D. B. Rensch and R. A. York, “A 120-Watt X-Band Spatially Combined Solid-State Amplifier”, IEEE Trans. Microwave Theory and Tech., vol. 47, (no. 12), IEEE, December 1999. p. 2557–61, a working active combiner unit using a slotline array inside an X band rectangular waveguide is disclosed. The bandwidth of the combiners is limited by the bandwidth of the rectangular waveguide, which has an fmax:fmin (maximum operational frequency over minimum operational frequency ratio) of less than 2. Since the dominant mode inside the rectangular waveguide is TE10 mode, the combiners also have a dispersion problem over the whole waveguide band.
In another reference, Jinho Jeong, Youngwoo Kwon, Sunyoung Lee, Changyul Cheon, Sovero EA. “A 1.6 W Power Amplifier Module At 24 Ghz Using New Waveguide-Based Power Combining Structures,” 2000 IEEE MTT-S International Microwave Symposium Digest (Cat. No.00CH37017), IEEE, Part vol. 2, 2000, pp. 817–20 vol. 2. Piscataway, N.J., USA, there is proposed an antipodal finline structure with double antipodal finlines inside a rectangular waveguide. The antipodal finline provides no-bond-wire transition from waveguide finline to microstrip line. It simplifies the connection with commercial off-the-shelf (COTS) microwave monolithic integrated circuits (MMIC) which predominantly use microstrip lines. However, as in U.S. Pat. No. 5,736,908 and other prior art, the bandwidth of the system is limited by the rectangular waveguide used.
U.S. Pat. No. 5,920,240, issued to Alexanian et al., discloses a coaxial waveguide power combiner/splitter, which inserts slotline cards into the coaxial waveguide for power distribution and combining. In the combiner/splitter, power devices are mounted on the slotline cards and then slid into the waveguide. This arrangement suffers from serious heat dissipation issues, as it is difficult to remove heat effectively from the power devices to an outside heat sink since the heat spreads to the slotline card first, then conducts to the waveguide through the sliding contacts between the slotline card and the waveguide. Because the combiner is mainly used for high power amplifier design and active devices are mostly high power amplifiers, the amount of heat generated is considerably high. The heat increases the operation temperature and decreases the lifetime of the amplifiers dramatically. Moreover, it is difficult to connect outside DC bias into the active devices on the slotline cards, and to access the slotline cards generally, as these are disposed inside an enclosed waveguide structure.
Two other references (Pengcheng Jia, R. A. York, “Multi-Octave Spatial Power Combining in Oversized Coaxial Waveguide”, IEEE Trans. Microwave Theory and Tech, vol. 50, (no. 5), IEEE, May 2002. p. 1355–60) and (Pengcheng Jia, Lee-Yin Chen, Alexanian A, York R A. “Broad-Band High-Power Amplifier Using Spatial Power-Combining Technique.” IEEE Transactions on Microwave Theory & Techniques, vol. 51, no. 12, December 2003, pp. 2469–75. Publisher: IEEE, USA) propose a stacked tray approach for power combining inside a coaxial waveguide. A plurality of identical wedge-shaped trays are stacked to form a coaxial waveguide, providing DC paths in the middle of the tray. In the first reference, active devices are mounted on the slotline card and directly connected to the end of the slotlines. Even though a metal tray is added underneath the slotline card, the thermal resistance caused by many layers of material and junctions remains problematic when high power devices are used. Since bonding wires are used to connect from slotline to MMIC which is not on the same layer, the parasitic effect will deteriorate the performance at higher frequency band. Further, assembly complications and costs are high.
In the second reference, an improved design enables easy assembly with COTS MMICs by integrating slotline to microstrip baluns to the end of slotlines. This provides improved thermal management since the active devices are directly mounted on to the metal wedge shaped trays. However, the balun has a slotline stub at the end of the narrow slotline on the backside of the substrate and a microstrip line stub on the top side of the substrate. The centers of the two stubs require alignment on the same axis perpendicular to the surface of the substrate. The accurate back side-to-top side alignment requirement significantly complicates the manufacturing process. The balun also takes considerable surface area. The size of the balun depends on the lower cutoff frequency of the system. The lower the cutoff frequency, the bigger the balun is. Since the surface area on the slotline circuit is limited, the maximum operational frequency range demonstrated by an arrangement of this second reference is only from 6 to 18 GHz, a 3:1 fmax:fmin ratio.
The slotline card design without slotline to microstrip balun disclosed in U.S. Pat. No. 5,920,240, shows a broader bandwidth ratio. However, if the end of the slotline is mounted on metal trays, then its dominant mode is TE mode, a non-TEM mode and dispersive over broad bandwidth. To achieve broad bandwidth response, the slotline needs to match with standard MMIC input/output impedance, 50 Ohm. Since the slotline tends to have high characteristic impedance, the gap of the slotline will be as narrow as 1 to 2 mil. The slotline cards thus require high accuracy photo-lithography instead of the conventional PCB (printed circuit board) processes which can normally achieve a best gap width of 4 to 6 mil. For this reason, the slotline cards used in real systems shown in the above-cited references are all built on ceramics with highly accurate lithography. This increases costs dramatically, and since the ceramics are fragile, it raises significant reliability issues.